1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus.
2. Description of the Related Art
Usually, a solid-state imagepickup apparatus includes an optical black region (OB region) which is light shielded so as not to react with light in order to obtain a signal (black reference signal) serving as a reference signal for a signal level. An effective pixel signal is arithmetically processed with reference to the signal outputted from the OB region. Such a pixel is called OB pixel. When such a solid-state image pickup apparatus is used for a digital still camera, etc., a prolonged period exposure time causes dark currents to be accumulated in the pixel to increase so that the dark current values fluctuate for every pixel and the fluctuation of signals become large. Although the black reference signal is obtained by clamping the output from the OB region, due to the fluctuation of signals as mentioned above, the black reference signal for every row fluctuates, resulting in such an adverse effect that lateral stripe occur on an image screen.
Moreover, as a countermeasure when a light shielding film constituting the OB region has a defect which causes light transmission to occur, in Japanese Patent Application Laid-Open No. H03-240379 (Patent document 1), a solid-state image pickup apparatus is disclosed, in which an impurity region for accumulating electric charges is not formed in the OB region. Since there is no accumulation of electric charges due to the dark currents, in the OB region of the solid-state image pickup apparatus, the above-mentioned adverse effect is suppressed. However, since, if especially, the exposure time period becomes long, a difference between the original black reference signal of the effective pixel region and the signal of the OB region will occur, it is difficult to supply an exact black reference signal.
The countermeasure for such a problem is disclosed in Japanese Patent Application Laid-Open No. 2002-64196 (Patent document 2). In the patent document 2, a solid-state image pickup apparatus includes a first OB region which is formed by implanting impurities into a semiconductor substrate, and a second OB region of which semiconductor substrate is not implanted with impurities, as the OB region. A signal from an effective pixel region is clamped with reference to the analog signal outputted from the second OB region, converted into a digital signal, and after that the digital signal is further clamped with reference to the digital signal outputted from the first OB region. It is supposed that while stable clamping including no unnecessary dark current, can be performed in this manner, the influence of the dark currents on the signals of the effective pixel region can be corrected exactly.
However, according to the configuration of the Patent document 2, the difference in output between the first OB region and the second OB region is present by an amount of the output voltage due to the dark currents. Consequently, the dynamic range of a circuit performing analog clamping and an analog-to-digital converter, and the dynamic range of the digital output of the analog-to-digital converter are required to be larger by the amount of the difference due to the dark currents.
Moreover, according to the layout view of the effective pixel region and the OB region, the first OB region formed by implanting impurities into a semiconductor substrate is arranged only at a part of each row (a horizontal optical black region in the second half of each row). Thus, in order to obtain a stable output level in the head row of the effective pixel region, the layout is configured to perform clamping using only the second OB region, of which semiconductor substrate is not injected with impurities.
In view of the above-mentioned problems, the object of the present invention is to obtain a high quality image with no lateral line etc. without being affected by the fluctuation of the dark outputs, and signals having a specifically large or small dark output, called defects.